With reference to FIGS. 8 and 9, a reactor building gas treatment system of a conventional boiling water-type nuclear power plant will be outlined.
FIG. 8 shows an example of a conventional plant called ABWR. In FIG. 8, a core 1 is housed inside a reactor pressure vessel 2. The reactor pressure vessel 2 is housed inside a containment vessel 3. The inner portion of the containment vessel 3 is divided into a dry well 4, which houses the reactor pressure vessel 2, and a wet well 5. The wet well 5 contains a suppression pool 6. Above the suppression pool 6, a wet well gas phase portion 7 is formed.
The atmosphere in the containment vessel 3 is replaced with nitrogen in order to keep oxygen concentration low, in the case of a boiling water light water reactor. Based on the material thereof, containment vessels 3 are generally categorized into a steel containment vessel, a reinforced concrete containment vessel (RCCV), a steel concrete composite structure (SC structure) containment vessel (SCCV), and the like. In the case of RCCV, a steel liner is put on the inner surface. FIG. 8 shows an example of RCCV, which is used in ABWR.
On top of the containment vessel 3, a containment vessel head 8 made of steel is provided. The containment vessel head 8 is joined to the containment vessel 3 via a containment vessel head flange 9. The containment vessel head 8 can be detached at the time of refueling. On an outer peripheral portion of the containment vessel head 8, there is a space known as a reactor well 10. The reactor well 10 is a space formed by a reactor well sidewall 11, which surrounds the periphery of the containment vessel head 8 and extends upward, a reactor well bottom portion 12, which is connected to a lower end of the reactor well sidewall 11 in such a way as to support the reactor well sidewall 11, the containment vessel head 8, and a shield plug 13. The reactor well bottom portion 12 is part of the containment vessel 3 in the case of RCCV. However, the reactor well bottom portion 12 is part of shield concrete that surrounds the periphery of a steel containment vessel in the case of the steel containment vessel. In general, the reactor well 10 is circular in horizontal cross section. However, the reactor well 10 may be elliptical or polygonal.
Above the reactor well 10, the shield plug 13 is placed. The shield plug 13 is mainly made of concrete, and is divided into several blocks 13a. The reason is to lighten the weight of one block 13a. The function of the shield plug 13 is to block radiation generated during the operation of a reactor. The joint areas of blocks 13a are therefore formed into a stepwise pattern, thereby blocking radiation from leaking via a gap 13b between the blocks toward an upper area. The gap 13b between the joint areas of blocks 13a may be about 1 cm, for example. Therefore, when the reactor starts to operate, the air inside the reactor well 10 is heated and expands. Part of the air passes through the gap 13b into the upper area.
Outside the reactor well 10, an operation floor 14 is provided in such a way as to connect to an upper end of the reactor well sidewall 11. An upper portion of the operation floor 14 is covered with an operation floor area wall 14c, which is part of a reactor building 15, in such a way as to form an operation floor area 14a, which is part of the space inside the reactor building 15.
To the reactor pressure vessel 2, major penetrating pipes, such as main steam lines and feed water lines, are connected. These lines or pipes penetrate the containment vessel 3 and then the reactor building 15, before being connected to a turbine and a main condenser inside a turbine building (not shown). Those major pipes, such as the main steam lines and the feed water lines, are collectively referred to as penetrating pipe 16 in the figures.
On the penetrating pipe 16, a first isolation valve (penetrating pipe isolation valve) 17 and a second isolation valve (penetrating pipe isolation valve) 18 are placed near the wall surfaces of the containment vessel 3. The figure shows an example in which the first isolation valve 17 is placed near the inner wall surface of the containment vessel 3, and the second isolation valve 18 is placed near the outer wall surface of the containment vessel 3. However, both of the two valves may be placed outside the containment vessel 3 in some cases.
If radioactive materials are released into the inner portion of the containment vessel 3 in the event of a design basis accident such as a loss-of-coolant accident, the isolation valves are automatically closed in order to prevent the leak of radioactive materials via the penetrating pipe 16 to the outside as much as possible. However, there is a design leakage rate set for the isolation valves, meaning that very small amount of radioactive materials would leak to the outside. Moreover, there is a design leakage rate (e.g., 0.4%/d in the example of ABWR) set for the containment vessel 3, meaning that very small amount of radioactive materials would leak from inside the containment vessel 3 into the inside of the reactor building 15.
In the reactor building 15, a standby gas treatment system (SGTS) 19 is provided. The standby gas treatment system 19 is designed to take in radioactive materials that are leaked into the reactor building 15 along with the atmosphere inside the reactor building 15, and removes the radioactive materials through a filter, and then releases the mainly decontaminated air into the environment from a high position. The standby gas treatment system 19 includes many branched suction pipes 20, an exhaust fan 21, a filter (filter train) 22, a standby gas treatment system exhaust pipe 23, and a heater 60. The heater 60 is disposed on the upstream side of the filter train 22. The standby gas treatment system 19 also includes isolation valves, which are not shown in the figure.
Inside the filter train 22, a charcoal filter filled with activated carbon is housed. The charcoal filter is capable of removing 99% or more of radioactive materials such as cesium iodide (CsI), for example. However, if the charcoal filter is wet, its performance becomes deteriorated. Accordingly, the atmosphere needs to be heated by the heater 60 in advance in order to limit the moisture. The standby gas treatment system exhaust pipe 23 is led into a stack 24 so that gas is released from an upper end thereof.
Inside the stack 24, the standby gas treatment system exhaust pipe 23 extends upward, thereby forming a double-tube structure, which is made up of the standby gas treatment system exhaust pipe 23 and the stack 24.
The exhaust fan 21, the heater 60, and the isolation valves of the standby gas treatment system 19 require an electric power source to operate. In the event of a design basis accident, electric power is supplied from an emergency DG (diesel generator) 25.
However, in the accident at the Fukushima Daiichi nuclear power plant, the offsite power was lost due to the earthquake and tsunami. Moreover, all emergency DGs 25 failed, and the system could not receive any supply of power from AC power sources, which is known as station blackout (SBO). The standby gas treatment system 19 therefore could not operate. Moreover, the core 1 could not be sufficiently cooled, resulting in a core melt accident. The cladding tube of the melted core fuel reacted with high temperature water, and the metal-water reaction generated large amounts of hydrogen, and the inside of the containment vessel 3 was over-pressurized.
In such a severe accident, the cooling of the containment vessel 3 could be insufficient, and the atmosphere inside the containment vessel 3 could become high in temperature, probably causing damage to the containment vessel head flange 9. As a result, hydrogen could leak into the reactor well 10 via the containment vessel head flange 9, and then into the operation floor area 14a via the gap 13b of the shield plug 13.
Moreover, a penetrating portion of the penetrating pipes 16 or hatch (not shown) portion could deteriorate at high temperatures, causing hydrogen to leak into the reactor building 15. Then, the hydrogen could rise up due to buoyancy, and be accumulated inside the operation floor area 14a. Because part of the operation floor 14 has an opening, such as staircase (not shown) the hydrogen can get into the operation floor area 14a via the opening. Then, the detonation of the hydrogen inside the operation floor area 14a caused damage to the reactor building 15.
In order to prevent such an event, an external water injection pipe 26 is provided so that water can be poured into the reactor well 10 from the outside. In the event of a severe accident, water can be poured from a fire truck 27 or the like in order to cool the containment vessel head flange 9. In this manner, new measures have been taken. Moreover, a new hydrogen vent system 28 is provided in the ceiling of the reactor building 15 so that the hydrogen accumulated in the operation floor area 14a can be released to the external environment.
Although the above description is for the containment vessel 3 and the reactor building 15 of an ABWR, those basic features are identical to those of conventional BWR/2, BWR/3, BWR/4, and BWR/5, which have been available prior to ABWR.
With reference to FIG. 9, an example of a conventional passive safety BWR, which uses a passive safety system, will be described. The conventional passive safety BWR includes passive cooling system pools 30a and 30b that keep cooling water above a containment vessel 3. In many cases, the passive cooling system pools 30a and 30b are connected together via a communicating pipe (not shown) so that the cooling water can move therebetween. Inside the passive cooling system pools 30a and 30b, a passive containment cooling system heat exchanger (PCCS Hx) 31a and a reactor isolation cooling system heat exchanger (IC Hx) 31b are provided. The PCCS Hx 31a cools the steam that is released into the containment vessel 3 in the event of an accident, and sends condensed water back into the containment vessel 3. The IC Hx 31b cools the steam inside the reactor pressure vessel 2 in the event of reactor isolation or an accident, and sends condensed water back into the reactor pressure vessel 2.
The heat that is generated at a time when the steam is cooled by the PCCS Hx 31a or the IC Hx 31b is transferred to the cooling water inside the passive cooling system pools 30a and 30b. After a certain period of time, the cooling water becomes so high enough in temperature that the cooling water starts boiling. The steam generated by the boiling of the cooling water is released to the external environment via exhaust ports 32a and 32b, which are provided in upper portions of the passive cooling system pools 30a and 30b. In many cases, the tips of the exhaust ports 32a and 32b are equipped with insect screens (not shown) in order to prevent insects and the like from getting into from the outside.
The upper portions of the passive cooling system pools 30a and 30b are covered with an operation floor 14. In a reactor well 10, shielding water 33 is always stored during normal operation. Radiation shielding effect of the shielding water 33 is almost equal to that of the shield plug 13 (FIG. 8). Therefore, no shield plug is placed. Above the operation floor 14 is an operation floor area 14a. The portion of the reactor building 15 (operation floor area wall) that covers an upper portion of the operation floor area 14a may be dome-shaped, as shown in FIG. 9. In such a case, the operation floor area wall is referred to as an operation floor dome 14b. In many cases, the reactor building 15 is built outside the operation floor dome 14b and the containment vessel 3, in such a way as to encircle the sidewalls of the containment vessel 3. In this case, as shown in FIG. 9, the operation floor area 14a makes up the space independent of a portion of the reactor building 15 that surrounds the sidewalls of the containment vessel 3.
Suction pipes 20 of a standby gas treatment system 19 are a large number of ramified pipes, which can take in the atmosphere from the operation floor area 14a inside the operation floor dome 14b as well as from other parts in the reactor building 15.
In another example of the passive safety BWR, the containment vessel 3, the passive cooling system pools 30a and 30b, and the operation floor area 14a may be housed in a reactor building (not shown) whose structure is the same as the reactor building 15 of ABWR (See FIG. 8). Even in this case, the outlets of the exhaust ports 32a and 32b of the passive cooling system pools lead to the environment outside the reactor building 15. However, there is a case where a standby gas treatment system is not provided like ESBWR (Economic Simplified Boiling Water Reactor) whose safety systems consists only of a passive safety system.
As an example of the reactor building gas treatment system for reactor accident, for example, the technology disclosed in Patent Document 1 (Japanese Patent Application Laid-Open Publication No. 2005-43131; the entire content of which is incorporated herein by reference) is known.